The invention relates to a method for the production of coated glass with a high transmissivity in the visible spectral range and with a high reflectivity for thermal radiation and a low surface resistance.
Such coated glass must admit a maximum percentage of the visible light and block a maximum percentage of the invisible spectral portion of solar radiation. In summertime this is intended to prevent the undesirable thermal radiation of sunlight from entering rooms. Especially in motor vehicles this effect is especially undesired on account of the sloping and therefore large front windshields and back windows. DE-OS 33 07 661 and EP-OS 104 870 to which respective U.S. Pat. No. 4,548,691 to Dietrich et al and 4,462,883 to Hart correspond, disclose sandwiching a silver layer and a very thin metal coating applied to the silver between two oxide layers. The thin metal coating protects the silver coating against chemical and thermal attack in the production of the outermost oxide coating. If such a sandwich, however, is exposed to temperatures above 150.degree. C., the silver diffuses into the adjacent oxide and/or metal coating, and a great increase in the surface resistivity and a corresponding reduction of the transmissivity of the sandwich is to be observed, i.e., two of the important properties of the sandwich are impaired.
EP-PS 35 906 to which U.S. Pat. No. 4,413,877 to Suzuki et al corresponds, teaches the arrangement of a thin metal coating on one or both sides of a silver coating and the embedding of this sandwich also between two oxide layers. The thin metal layer is intended to achieve a greater long-term stability, although the experiments are limited to a temperature range up to 120.degree. C. Since furthermore the absorptivity of the metal layers results in an impairment of the optical transparency of the coated substrates, the total thickness of the metal layers for the achievement of a particular transmissivity is limited, and therefore preferably only a single metal coating is to be applied to the side of the silver coating facing away from the substrate so as to limit the impairment of the transmissivity. It has been found, however, in the use of substrates of mineral glass and a system of layered coatings of this kind, that, in the event of heating to temperatures markedly above 150.degree. C., an impairment of the coating properties also occurred, not only by a desirable elevation of the transmissivity in the visible range, but at the same time by an undesired increase in the surface resistance, so that the on-board voltage of a normal vehicle no longer suffices to produce enough heating in winter. Furthermore, spotting and undesirable light scatter have been observed, which are evidently due to a partial agglomeration and to a partial diffusion of the silver into the oxide layers.
This makes it possible only to conclude that the thickness of the metal layer or layers aimed at the desired high transmissivity in the visible range, which is to be attributed in part to motor vehicle regulations concerning motor vehicle windows, is no longer sufficient for the effective protection of the silver coating, especially at relatively high temperatures.
It is also known to produce curved or domed glazing with approximately similar spectral behavior by applying a silver coating embedded in other coatings to previously shaped glass substrates. This, however, requires special coating apparatus to provide for the different shapes of the substrates and by complex masking arrangements to compensate for the influence of different distances between elements of the area of the substrate and the coating sources and different angles of incidence on the coating material.
Lastly, by a method of an older patent application (P 35 43 178.4), to which U.S. Pat. No. 4,919,778 to Dietrich et al corresponds, substrates of mineral glass can be provided, by means of a vacuum coating process, with a five-layer system whereby the coated glass is given a high reflectivity for thermal radiation combined with a simultaneous high transmissivity for visible light. The five-layer system is selected so that the initially flat, coated glass can be subjected to a bending process in which the softening temperature of the glass of about 650.degree. C. is reached, without altering negatively the properties of the coated glass. The coated glass thus treated furthermore has a low electrical resistance due to the coating, so that it can be heated by the application of an electrical current source.
When such coated glass are combined with another uncoated glass in a sandwich-like construction, they are especially suitable for use in motor vehicles as heatable front or rear windows having an additional function of protecting against intense solar radiation. In such sandwiched coated glass coated glass (laminated safety glass) a tough elastic interlayer of a plastic (e.g., polyvinylbutyrate) is placed between the two thin sheets of mineral glass with the exclusion of air. In the case of the coated glass according to the U.S. Pat. No. 4,919,778, the coated side is in direct contact with the plastic. The purpose of the plastic interlayer is to bond the two sheets permanently together in a stable unit which has a decidedly higher resistance to breakage than a single sheet. For this purpose a certain adhesion of the plastic to the glass or coating surface is necessary. Furthermore, the strong bonding of the tough plastic to the glass sheet is intended to prevent large glass splinters from being formed and scattered about in case the window is broken, in the event of an accident, for example, so as to minimize injuries to the vehicle occupants. To provide the assurance of such properties, certain test rules have been established and must be observed for the use of such sandwiched coated glassin motor vehicles. These rules and test methods are laid down in standards. In performing these tests on laminated safety glass which have been made by the method given in the older patent application, it was found that the bond of the sandwich system is insufficient, since the glass fragments would not adhere to the (elastic) plastic film when the window was broken.
It is therefore the object of the present invention to create a series of layers which will lead to a substantially improved adhesion of the series of coatings to one another without impairing the other physical data necessary for the planned application, especially the high light transmission, the electrical conductivity and the stability of the sandwich during the bending process.